It is now recommended that levothyroxine doses be reduced when the drug is used to suppress thyroid-stimulating hormone (TSH), so that a thyrotoxicosis-induced increased risk for osteoporosis is minimized in patients with a history of thyroid cancer who have been apparently tumor-free for many years [1-5]. However, no data address whether low but detectable TSH levels can achieve the clinical benefit that was seen when TSH was extinguished. Furthermore, a tumor-free state is difficult to definitively ascertain.

We are concerned that some patients with a history of thyroid cancer may be harmed when TSH is unclamped unless they are first evaluated for dormant and unsuspected persistent or recurrent tumor. This concern was heightened by our recent discovery, incidental to a plan to reduce the levothyroxine dose, of unmanifested thyroid cancer that was metastatic to the lung in a 57-year-old woman with asymptomatic osteoporosis and TSH suppression. Thirty years before, papillary thyroid carcinoma had been treated surgically, followed by I131 scanning. Whole-body scans in 1965, 1973, and 1981 were negative. The current thyroid sonogram, chest radiograph, and magnetic resonance imaging scan were physiologic, and the test for thyroglobulin (border-line high level) was invalid because of interfering antibodies. Withdrawal of levothyroxine, hypothyroidism, and a whole-body I131 scan were needed to reveal the metastatic cancer (Figure 1).